Electrical signals travel very fast but their speed is not
infinite. Just how fast depends on the characteristics of the conductor.
Transmission lines can be considered to be very uniform conductors; the
capacitance and inductance per unit of length is constant over the whole length
of the line. Each infinitesimally small series inductance must have its related
magnetic field developed and each small parallel capacitor must be charged. All
of this takes time and so a signal in a transmission line is delayed compared
to the time a radio signal in free space would take to travel the same
distance. This delay, the ratio of the speed of transmission of the signal to
the speed of light, is called the velocity factor.
The velocity factor is a physical characteristic of the line and
depends primarily on the dielectric used to construct the line. For parallel
conductor lines using air dielectric the velocity factor is about 0.98; for
polyethylene coaxial cable it is about 0.66. Foam dielectric coaxial cables
have air bubbles in the polyethylene and have velocity factors from about 0.70
to 0.85.
Velocity factors are always provided in cable catalogues and also
can be determined from most standard reference manuals such as Bill Orr's The Radio Handbook. If you know the
dielectric constant for the material separating the two conductors in a
transmission line the velocity factor can be calculated using the following
relationship:
1
Velocity Factor =
----------------------------
√ (Dielectric Constant)
This says that the Velocity Factor for a particular transmission
line is equal to the reciprocal of the square root of the dielectric constant
of the material between the two conductors.
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